The tunnels are a key component in extending the Muni Metro T Third Line through SoMa, Union Square and Chinatown.

The 350-foot-long, 750-ton machine began its journey under 4th Street. During the next 10 months, Mom Chung will travel north under 4th Street, Stockton Street and Columbus Avenue, excavating and constructing San Francisco's first new subway tunnel in decades.

An identical machine, named Big Alma, has arrived in San Francisco to construct a tunnel parallel to Mom Chung's. After about six weeks of assembly underground, Big Alma will begin tunneling.

"We are one step closer to building the 21st Century transportation system our world-class city needs," said San Francisco Mayor Edwin Lee. "The Central Subway and its tunnels are essential to building and maintaining a reliable, modern public transportation system for San Francisco residents and connecting our diverse communities."

Mom Chung and Big Alma will excavate and construct the 1.5-mile-long tunnels at a pace of approximately 40 feet per day, though their pace will vary based on ground conditions and other factors. Most of their journey will be through two major ground formations: the Franciscan complex, a bedrock formation that forms Nob Hill and the Colma formation, a dense mixture of sand and clay. The TBMs will be so far beneath the surface, between 40 and 120 feet underground, that no vibration or noise will be felt above ground when they pass below.